1. Field of the Invention
The present invention relates to a nonvolatile memory device. In particular, the invention relates to a random-access memory including memory cells having magnetic tunnel junctions (MTJ).
2. Description of the Background Art
In recent years, an MRAM (Magnetic Random-Access Memory) device has been of great interest as a new-generation nonvolatile memory device. The MRAM device is a nonvolatile memory device in which a plurality of thin-film magnetic elements formed in a semiconductor integrated circuit are used to store data in nonvolatile manner, and the thin-film magnetic elements are each randomly accessible. In particular, recently it has been known that an MRAM device is remarkably improved in performance by employment of the thin-film magnetic element using a magnetic tunnel junction (MTJ) as a memory cell.
Generally, when data is to be read from a memory cell configured using such a thin-film magnetic element as described above, an electric current flowing through a tunneling magneto-resistance element (TMR element) therein or an end-to-end voltage of the TMR element can be measured to indirectly measure the electric resistance value of the TMR element and thereby read data.
It is now a particularly important challenge to reduce the cell area of the memory cell in order that such an MRAM device as described above may replace a main memory using a DRAM (Dynamic Random Access Memory) device.
However, supposing that a theoretical minimum cell area of a memory cell of a DRAM device which is now commonly used is 6F2, the theoretical minimum memory cell area of a memory cell of an MRAM device configured with one transistor and one TMR element is 12F2. Namely, the MRAM device requires the cell area twice as large as that of the DRAM device. This is for the reason that the memory cell of the MRAM device additionally requires word lines used for reading data as compared with the memory cell of the DRAM device.
Therefore, a memory cell based on the spin injection technique has recently been proposed that can accomplish a cell area equivalent to the cell area of the DRAM device (for example, Japanese Patent Laying-Open Nos. 2005-011907, 2004-111904 and 2005-092912). The memory cell based on the spin-injection technique differs from the currently-used MRAM device in terms of how data is written. For a memory cell of the currently-used MRAM device, a method is employed of allowing an electric current to flow through lines (including write word line) adjacent to a TMR element to generate magnetic fields and thereby reverse the direction of magnetization. In contrast, for a memory cell based on the spin-injection technique, a method is employed according to which an electric current is applied directly through a TMR element to reverse the direction of magnetization of the TMR element. The electric-current flow direction is changed to switch the direction of magnetization of a free layer to the parallel or antiparallel direction with respect to a fixed layer. This method is called the spin injection technique since the direction of magnetization is reversed using the action of spin-polarized electrons in the current. By-employing the spin injection technique, the theoretical memory cell area of the MRAM device can be reduced to a half of that of the currently-used MRAM device.
The method of reading data of the MRAM device based on the spin injection technique is similar to that of the currently-used MRAM device. Specifically, an electric current flowing through a tunneling magneto-resistance element (TMR element) with which a memory cell is configured or an end-to-end voltage of the TMR element is measured to indirectly measure the electric resistance value of the TMR element and thereby read data. Therefore, in the MRAM device based on the spin injection technique, an electric current is applied to the TMR element in both of the data write and data read operations. Therefore, if the read current has a large value, an accidental data write occurs, resulting in a problem that the stored data is corrupted. This phenomenon is called “read disturb.” If the read current value is decreased for avoiding the read disturb, the sense voltage level is also decreased to cause the problem that the data read speed, namely the access speed decreases.
When the temperature of the TMR element itself increases, the free layer becomes unstable and reversal of the magnetization direction is likely to occur. This phenomenon is called “thermal assist effect.” The thermal assist effect largely depends on the electric-current supply duration for which the electric current is supplied. Therefore, even for the same read or write current, a longer electric-current supply duration leads to a higher possibility that reversal of the magnetization direction occurs.
Regarding the currently-used MRAM device as well, if the time for which a write magnetic field is applied and the time for which a read current is supplied from lines adjacent to the TMR element are longer, the temperature of the TMR element itself increases. A problem thus arises is that the reliability of storage data of a TMR element, namely the thermal disturbance resistance against the ambient temperature deteriorates.